The invention is directed to a process for the production of coated nuclear fuel particles by batchwise deposition of pyrolytic carbon and/or silicon carbide on fuel particles and subsequent treatment by sieving and/or sizing (i.e. classifying) the coated particles.
The fuel elements for gas coated high temperature reactors (HTR) contain the fuel in the form of coated particles. The coated particles consist of a spherical fuel nucleus which is jacketed several times by pyrolytic carbon layers alone or in combination with silicon carbide. As nuclear fuel there are employed uranium 235 and fissionable plutonium isotopes in the form of the carbide or oxide while as fertile material there are employed thorium 232 and uranium 238 (e.g., as the carbide or oxide). At a nucleus diameter of 100-600 microns the total diameter of the particles is 400-1200 microns. The layers of pyrolytic carbon alone or in combination with silicon carbide have a total thickness of 120-300 microns.
The coating of the fuel muclei has the task of retaining inside the individual fuel particles the radioactive fission products which are formed during the operation of the reactor. From this there results the requirement that the coating is not permitted to suffer any damage and therewith the gaseous and solid fission products are retained to a sufficient extent.
The coated particles are burdened with several stresses. Thus with increasing burn-up the fission gas content increases and therewith the pressure in the inside of the particles increases. The target value for the average burn-up with the high temperature reactors is relatively high and according to the type of reactor amounts to 75,000 to 100,000 MWd/t.* This high burn-up together with the maximum fuel temperature laid out of about 1350.degree. C. levels to a very high fission gas pressure which in the interior of the particles can exceed the value of 100 bar. FNT *(megawatt days per ton uranium and thorium)
Furthermore there are formed thermal stresses which depend essentially upon the particle power, temperature and the physical properties of the individual layers which in return are co-determining for temperature gradients built up in these layers.
Besides radiation induced stresses occur since with the irradiation with quick neutrons the pyrolytic carbon layers begin to shrink. The shrinking increases with increasing fluency of fast neutrons and produces radiation induced stresses in the layers which can only be reduced by creep processes.
In contrast to this shrinking process the fuel nucleus begins to expand because of the fission gas pressure building up. In order to meet the counteracting dimensional charges so far that no inadmissible stresses occur the pyrolytic carbon coating always contains, both alone as well as in combination with silicon carbide, a porous inner layer (buffer layer) which makes accessible empty volumes for fission gas pressure and swelling of the nucleus (U.S. Pat. No. 3,325,363 and Goeddel German Patent No. 1,471,183). The thickness of the buffer layer and with it the accessible empty volumes is so calculated that it satisfies both of the requirements for burn-up and fluency of fast neutrons with the given particle sizes and fuel temperature (J. W. Prados and I. L. Scott, Nuclear Application, Vol. 2, 1966, p. 402). The outer coating which is built up from one or more high density layers is responsible for the actual retention of gaseous and solid fission products (German Patent Nos. 1,571,518 and 1,915,670).
The coating of the fuel nuclei with pyrolytic carbon and silicon carbide generally takes place in fluidized bed apparatuses. These apparatuses consist of a vertical, heated graphite tube with a conical bottom. One or more nozzles discharge into the apex of the cone, through which the carrier gas, e.g. argon or helium, necessary for the fluidizing and coating gas are blown in. The pyrolytic carbon layers are deposited through thermal decomposition of methane, acetylene, propane (and other gaseous aliphatic hydrocarbons) from the gas phase at temperatures between 1200.degree. and 2100.degree. C. In the coating with silicon carbide as the coating gas there is preferably employed methyl-trichlorosilane. Depending on the coating conditions there are obtained layers of different density and structure with different physical and mechanical properties. In order to separate faulty coated particles, the coated particles are sieved and/or unround particles are sorted out (i.e., the particles are classified) on a vibrating chute. The size of the charge of the fluidized bed apparatuses for coating on a production scale is relatively large and can amount to up to about 20 kg of heavy metal. This corresponds, according to the size of the nucleus to a very high number of particles, i.e., from about 7 to 30 million particles.
In order to be able to produce economically the porous pyrolytic carbon layers with the necessary properties they must be deposited with a very high growth rate of about 6 microns/min. Compared to the growth rate for dense outer layers this is about a factor of 10 higher.
Based on the large number of particles and relatively high growth rate therefore it cannot be avoided that despite constant coating conditions a certain portion of coated particles in the buffer layer have undesirably high deviations. As a result there are found particles both with extreme thickness as well as with unsuitably thin buffer layers. Since the porous buffer layer has a low heat conductivity, the temperature drop increases strongly in undesirably thick buffer layers with the irradiation which results in an increase of the fuel temperature. Through this inside these particles the chemical stability is influenced unfavorably the diffusion rate of the fission products increases and the stress load increased.
In contrast with the particles with insufficiently thin buffer layers the accessible empty volumes is not sufficient to collect the burn-up caused swelling of the nucleus and the gaseous fission products liberated. Therefore stresses can occur especially at high burn-up which lead to the breaking of particles and bring about the setting free of fission products.
The reason for the increasingly defective part in the operation of the nuclear reactor above all is the quality of the coating, of which a particularly predominant reason, however, is the portion of the particles with extremely thick or extremely thin buffer layers.
The requirement for lower activity contamination of the primary circuit is especially sought with advanced types of reactors as, e.g., single circuit plants with helium-turbine and nuclear process heat-reactors. The target value sought can then only be attained if the defective portion of the ascertained particles for operation of the reactor is clearly reduced.